1. Field of the Invention
The present invention relates to a technology that is well adaptable to an electric storage device including plural positive-electrode mixture layers of different types.
2. Description of the Related Arts
A high energy density and high output density are demanded to an electric storage device that is mounted to an electric vehicle, hybrid vehicle, or the like, or to an electric storage device installed into various power tools. Therefore, a lithium ion secondary battery, an electric double layer capacitor, etc. have been nominated as a candidate for the electric storage device. However, the lithium ion secondary battery has a high energy density, but low output density. The electric double layer capacitor has a high output density, but low energy density.
In view of this, there has been proposed an electric storage device called a hybrid capacitor in which the electric storage principles of the lithium ion secondary battery and the electric double layer capacitor are combined in order to achieve both of the satisfactory energy density and the satisfactory output density. The hybrid capacitor employs an activated carbon, which is used for the electric double layer capacitor, for a positive electrode so as to accumulate charges by utilizing the electric double layer in the positive electrode, and employs a carbon material, which is used for a lithium ion secondary battery, for a negative electrode, and lithium ions are doped into the carbon material of the negative electrode so as to accumulate charges. The application of the electric storage mechanism described above makes it possible to enhance the output density and the energy density. However, a further improvement in the output density and the energy density has been demanded in order to use the electric storage mechanism described above for a vehicle power source or a power source for a power tool.
Methods for enhancing an output density of a battery having a high energy density include the one in which an internal resistance is reduced by coating an electrode mixture material to be thin or by increasing an electrode area, and the one in which a battery and a capacitor are connected in parallel in order to supply great electric current from the capacitor. However, in the former method, the electrode mixture material is coated to be thin, which entails a reduction in the energy density of the electric storage device, or which makes the assembly difficult to thereby increase cost of the electric storage device. In the latter method, the battery and the capacitor are combined, which entails a reduction in the energy density of the whole electric storage device, or which entails increased cost of the electric storage device due to a complicated control circuit.
In order to solve these problems, there has been proposed an electric storage device in which positive-electrode current collectors of a lithium ion secondary battery and an electric double layer capacitor are connected to each other, and negative-electrode current collectors of the lithium ion secondary battery and the electric double layer capacitor are connected to each other (e.g., see Japanese Patent Application Laid-Open No. 2001-351688). Further, an electric storage device has been proposed in which a mixture material including an activated carbon or the like and a mixture material including a lithium cobalt oxide or the like are coated on a single current collector in a overlapping manner (e.g., see Japanese Patent Application Laid-Open No. 2000-36325 and Japanese Patent Application Laid-Open No. 2005-203131), or an electric storage device has been proposed in which a mixture material having mixed therein an activated carbon and a lithium cobalt oxide is coated on a single current collector (e.g., see International Publication WO 2002/41420).
However, in the electric storage device disclosed in Japanese Patent Application Laid-Open No. 2001-351688, it is difficult to cancel the difference in the potential between the electrodes that are connected to each other. Therefore, over discharge or overcharge of the lithium ion secondary battery or the electric double layer capacitor may occur. The occurrence of the over discharge or overcharge described above causes the deterioration in durability of the electric storage device. In the electric storage devices disclosed in Japanese Patent Application Laid-Open No. 2000-36325, Japanese Patent Application Laid-Open No. 2005-203131, and International Publication WO 2002/41420, it is difficult to secure the output density by sufficiently reducing the internal resistance. Further, the electric storage devices have a structure in which the lithium cobalt oxide is in contact with the activated carbon. Therefore, the affect caused by the deteriorated lithium cobalt oxide also affects the activated carbon, which deteriorates the durability of the electric storage device.